Screens are used in the aggregate business for separating rock, crushed rock, gravel, sand, and the like (referred herein as material) into various component sizes, referred to as size fractions. Screens comprise one or more screen decks containing a perforated screening medium which acts as a sieve through which the material is separated. A charge of material is deposited on the receiving end of the screen, and as the material is conveyed to the discharge end, smaller material falls through the openings leaving the larger material behind.
In a common application in the production of gravel, such as for road building, at the quarry site, a charge of material is crushed using a rock crusher. The crushed material is then conveyed to the screen for separating. In an example of the use of a three-deck screen, material is separated into four sizes: large, medium, small, and smallest. The larger material is retained on the upper screen deck and conveyed off of the screen deck at the upper discharge end, the medium-sized material is retained on the middle screen deck and conveyed off of the screen deck at the middle discharge end, the smaller size material is retained on the lower screen deck and conveyed off of the screen deck at the lower discharge end, and the smallest material is deposited below the lower screen deck. The larger material, if too large for a particular purpose, may be collected from the screen and reprocessed by the crusher and re-screened until the desired size is obtained. Screens are commonly very large machines that are capable of continuously separating large quantities of material, hundreds of tons per hour, as part of the quarry operation.
There are various types of screens loosely classified by the configuration of the screen deck and the method used to pass the material through the screening medium. One common method to pass the material through the screening medium is to submit the screen deck to vibratory motion to agitate and expose the material to the screening medium surface. The screens have a front or receiving end that receives the mixed material and a back or discharge end that discharges the separated material.
The screen deck generally consists of a rigid frame upon which a screening medium is laid or supported. The screening medium contains a plurality of openings of a predetermined size. Examples of screening medium include woven wire cloth and perforated plate. Material is placed upon the screening medium and material that is smaller than the predetermined size falls through the openings in the screening medium, and thus separates the smaller material from the larger material. The material that is larger than the predetermined size of the openings is subsequently removed from the screen deck, and commonly made to move across the screen deck to be discharged at a location separate from the smaller material. The capability of the screen to convey the material in combination with screening allows for continuous material processing.
Screens come in two basic screen deck configurations; inclined and horizontal. Inclined screens have one or more screen decks with an elevated receiving end with respect to the discharge end. Material is placed on the higher end of the screen deck, and as the material moves down the inclined screen deck to the discharge end, the smaller material passes through the openings of the screening medium. The larger material is discharged from the screen deck at the discharge end.
The movement of material down the screen deck is provided by gravity, or, more commonly, in combination with the assistance of a vibrating mechanism. The vibrating mechanism is not only used to assist gravity, but also to agitate the material to more efficiently present the smaller material to the screening medium.
Quarry-sized inclined screens are very tall machines. Being such tall machines, inclined screens are difficult to transport from quarry to quarry. When transportation is required, inclined screens are commonly disassembled and broken down requiring significant labor and time for both disassembly and re-assembly.
Horizontal screens are configured such that the screen deck is level or horizontal. Horizontal screens are normally selected when there is a need to maintain a lower profile, such as for use in confined spaces or for transportation/mobility considerations. Horizontal screens require the use of a vibrating mechanism to agitate the material for effective separation. The vibrating mechanism is configured in its construction and operation to not only agitate the material, but also convey the material from the receiving end to the discharge end in screens having a continuous material processing capability. Horizontal screens require significantly more powerful and aggressive vibrating mechanisms to agitate and convey the material along the screen deck as compared with the inclined screen.
It is common that screens utilize a plurality of screen decks in a stacked arrangement, one above the other, to separate the material into multiple sizes. In the case of a three-deck screen with an upper, middle and lower screen deck, the upper screen deck comprises the largest openings, the middle screen deck comprises smaller openings, and the lower screen deck comprises the smallest openings. As the material traverses the upper screen deck, the larger material remains on the upper screen deck while the smaller material falls to the middle screen deck. The middle screen deck with the smaller openings contains the medium sized material while allowing the passage of smaller material to the lower screen deck. The lower screen deck with the smallest openings contains the smaller material while allowing the smallest material, such as dust or fines, to pass through. As the separated material is conveyed along its respective screen deck to the discharge end, it is deposited into four separate areas for collection; large, medium, small, and smallest material size fractions. The three-deck screen, therefore, is capable of separating material into four material size fractions.
In operation, the multiple-deck screen will deposit material onto the underlying screen decks at different rates and locations. For example, the material that passes through the upper screen deck will fall to the middle screen deck somewhat down-line from the receiving end of the upper screen deck. In like fashion, the material that passes through the middle screen deck will fall to the lower screen deck somewhat further down-line from the receiving end of the upper screen deck. The delay in dropping the material through the screen decks is due to the fact that the particles must transcend down through the layer of material, referred to as the material bed, on one screen deck before it can drop through to the screen deck below. Therefore, the length of the screen depends on the number of screen decks and the relative speed that the material passes through each subsequent screen deck.
It is common for screens to utilize a vibrating mechanism to assist in the separation process as well as in the conveyance of the material towards the discharge end. The one or more screen decks are coupled together to a common rigid frame. The assembly comprising the multiple screen decks and the common frame is known as the screen box. The screen box is vibrated by a vibrating mechanism that is coupled to the common frame. Therefore, one vibrating mechanism vibrates all the screen decks simultaneously. The vibratory motions promote stratification in the material bed, bringing the smaller material down to the screening medium surface to be passed through the openings.
The common types of vibrating mechanisms can be characterized by the form of the vibration and the number of bearings used in the mechanism. A two bearing, circle throw, inclined screen utilizes a counter weight on a shaft to vibrate the screen box, and therefore the screen decks, in a desired motion. Common vibrating mechanisms produce motions that include circular, elliptic and straight-line reciprocating movement. The motion can be directed to propel the material toward the discharge end to help convey the material in that direction. The screen box is isolated from the ground or support structure by springs or other damping apparatus.
Separation efficiency is determined in part by the operating parameters of the vibrating mechanism. Those parameters include frequency, amplitude, attack angle and travel velocity imparted on the material. For a given material size distribution, weight, shape and quantity, as well as size of the openings, an optimum set of parameters can be determined for a given screen deck. Since a common vibrating mechanism is used to vibrate all of the screen decks simultaneously, the parameters set on the vibrating mechanism for multi-deck screens will be a compromise of efficiency for any one particular screen deck.
The efficiency of operation of screens is determined in part by the power required to separate a given quantity of material. The power to operate an inclined screen includes the power to lift the material to the height of the receiving end of the screen, as well as the power used to move the material across the screen decks. Inclined screens take advantage of gravity to convey the material towards the discharge end. In contrast, the horizontal screen power requirement is potentially less to load the material onto the receiving end, but is significantly more to move the material along the screen deck.
The screening medium surface is the most life-limited part of a screen. The screening medium surface must be strong enough to withstand the initial impact of the bulk material onto the receiving end of the screen deck as well as the material falling on the lower screen decks. The screening medium surface must also support the weight of the material and be flexible enough to withstand the vibration. Additionally, the screening medium must provide enough open area to allow the desired throughput of material while preventing the openings from becoming clogged.
The above mentioned vibrating screens have a number of drawbacks. Regarding the inclined screens, the height of the screen is a significant hindrance for moving the screen from place to place. Most particularly, the inclined screens require disassembly in order to move them along improved roadways with overhead obstructions requiring significant labor and time.
Inclined screens are known to cause a “snowball” effect as the material is conveyed down the screen decks. That is, material placed on the receiving end of the screen deck is at first conveyed slowly down the screen deck but increases in speed and momentum sufficient to overcome the preceding material. This causes a piling up of material increasing the material bed depth. As the material bed depth increases, separation efficiency decreases as it takes longer for the smaller material to transcend the material bed and make contact with the screening medium surface.
Horizontal screens are more readily transportable but require considerable power to operate and move the material through the machine. Further, horizontal screens are limited to the number of screen decks, commonly three, that can be used. This is due to the length of screen deck required to pass the material through each subsequent screen deck, in part caused by the delay in material dropping from the screen decks above.
An improved screen is needed that incorporates the reduced height of a horizontal screen for improved transportability and reduced power requirements in lifting the material to the receiving end, with the power efficiencies of the inclined screen, while keeping the overall length of the screen to a minimum and decreasing the detrimental effects of the “snowball” effect. Improvements are also needed to increase the lifetime of the screening medium, particularly to reduce the damage caused by the initial impact loads of the material dropping onto the screen decks.